1. Field of the Invention
The invention relates to a real-time adaptive measuring method for determining the thermal conductivity K between a sensor and a medium. More particularly, the present invention is directed to determining thermal capacity C of a sensor and the temperature of a medium by means of the sensor when it is in a thermal exchange relation with the medium, by supplying thermal power to change the temperature of the sensor and by measuring the thermal power supplied to the sensor.
2. Description of Related Art
Known devices for measuring changes in thermal conductivity, for example, thermoanemometers, are based on the use of either one or two temperature elements. In devices utilizing two elements, one of the elements is heated and the power used for heating is simultaneously determined, whereas the other element is used to determine the temperature of the medium to be measured.
Devices utilizing two temperature elements fall into two groups. In one group, constant power is supplied to heat one element and the temperature difference between the two elements is measured. In a more common arrangement, the temperature difference between the elements is kept constant and the thermal power required for this purpose is measured. The latter method is advantageous in that the rate at which the device responds to changes in the quantity to be measured increases in proportion to the amplification of a feedback loop used for adjusting the temperature difference between the elements.
Devices utilizing one temperature element either measure the cooling or warming rate of the element or the thermal power required to maintain the sensor at a desired temperature. The former method corresponds to the method using two sensors, wherein constant power is supplied to heat one element and the temperature difference between the elements is measured. The latter method mainly corresponds to the method using two elements, wherein the temperature difference between the elements is kept constant and the thermal power required to keep it constant is measured. The problem with utilizing one element is that the temperature of the medium is not known, and so the power required to maintain the sensor at the desired temperature depends not only on the thermal conductivity of the medium but also, for example, on changes in the temperature of the medium, and these variables cannot be distinguished from one another.
U.S. Pat. No. 5,117,691 (the '691 patent) discloses a one-sensor measuring method that is based on differentiation and that operates without information about the surrounding temperature. In contrast, the invention according to the present application uses correlation checking, and determination of the temperature of the medium through calculation is an essential part of the method. Another significant difference from the method of the '691 patent is that the measurement takes place in real time. The method according to the present invention operates in real time, without the steady state that is required by the method described in the '691 patent and prevents its use for measurement in real time. The device according to the '691 patent can be implemented so that the temperature of the sensor need not stabilize and reach the steady state, but then the circuit uses a demodulator and measurement results still cannot be obtained in real time.
The problem with all the measuring devices utilizing two temperature sensors, such as these thermoanemometers, is the difficulty of maintaining their calibration over a wide temperature range. When a variable is determined using the temperature difference between two temperature sensors, their calibration must track very closely over the entire operating temperature range. Even when expensive precision components are used, this calibration still sometimes causes problems. For example the ageing of components, even the heat radiation or warming caused by the electronics themselves, can cause problems.
On the other hand, the problem with measuring devices utilizing one temperature element has been their slow response time. They are comparable to the conventional thermoanemometer that uses two temperature sensors and keeps the thermal power constant, measuring the temperature difference between the temperature sensors. In these conventional two-element thermoanemometers response time is shortened when the temperature difference between the temperature sensors is kept constant by adjusting the thermal power provided to heat the sensor. The speed of the thermoanemometer further increases in proportion to the amplification coefficient used in the adjustment. This has not been applicable lo devices having one sensor, since the temperature of the medium to be measured is not known and, as the temperature of the medium changes, keeping the temperature sensor at a constant temperature does not ensure that the temperature difference between the sensor and the medium remains constant.
In addition to their long response time, another problem with devices using one sensor is that in the method based on measuring the cooling rate of one sensor, the thermal capacity C of the sensor is assumed to be known, or at least constant. The signal-to-noise ratio is also poor, since the temperature difference between the sensor and the medium decreases during the measurement, but the noise level remains constant.